The invention relates to a process for purifying anhydrous industrial hydrogen fluoride.
Industrially manufactured hydrogen fluoride (CaF.sub.2 +H.sub.2 SO.sub.4 .fwdarw.2 HF+CaSO.sub.4) generally occurs as anhydrous hydrogen fluoride, which contains some impurities and small amounts of water, depending on the starting material (normally fluorspar).
The term "anhydrous hydrogen fluoride" is thus understood to mean a hydrogen fluoride as obtained by a conventional industrial process and having a water content of less than 5,000 mg of water/kg of hydrogen fluoride.
The impurities are substantially compounds of the elements arsenic, phosphorus, boron, silicon and sulphur.
These compounds cause problems in many applications, and are therefore undesirable.
When aqueous hydrogen fluoride solutions are utilised in the manufacture of electronic components, the compounds of the elements arsenic, phosphorus, silicon and boron, particularly, cause problems.
In applications in which hydrogen fluoride and a catalyst are utilised, for example in the manufacture of fluorocarbons--substances which replace chlorofluorocarbons--it is the sulphur compounds which particularly give rise to problems.
In U.S. Pat. No. 3,166,379 compounds of the elements arsenic, phosphorus and sulphur are removed by adding chlorine, bromine or iodine, followed by distillation of the hydrogen fluoride.
In U.S. Pat. No. 4,668,497 compounds of the elements boron, silicon, phosphorus, sulphur, chlorine and arsenic are removed by adding elemental fluorine, followed by distillation of the hydrogen fluoride.
In U.S. Pat. No. 4,032,621 compounds of the elements arsenic and sulphur are removed by adding permanganates or dichromates, followed by distillation of the hydrogen fluoride.
The disadvantage of the aforementioned processes is that the addition of oxidants, with the exception of fluorine, to the industrial hydrogen fluoride adds to the hydrogen fluoride an impurity not previously contained in it. Following the oxidation, the hydrogen fluoride must therefore be distilled in order to separate the oxidant and the oxidised impurities.
A disadvantage of using fluorine is that it entails handling problems.
Depending on the oxidant added, a reducing agent must moreover be added before distillation in order to destroy the excess oxidant (U.S. Pat. No. 4,032,621), thus drawing further impurities into the hydrogen fluoride.
In DE-A 4 031 967 and WO 91/13024 the hydrogen fluoride is purified by electrolysis on nickel or carbon electrodes. The disadvantage of these processes is that carbon electrodes have proved to be not particularly suitable because of their short service life in hydrogen fluoride electrolysis. The disadvantage of utilising nickel electrodes is the occurrence of severe electrode corrosion. This introduces nickel compounds as impurities into the hydrogen fluoride to be treated.
The object of the invention was therefore to provide a process for purifying anhydrous industrial hydrogen fluoride which does not have the disadvantages mentioned.